1. Field of the Invention
The present invention relates to an inverter control apparatus for controlling an inverter that interconnects with an alternating-current (AC) power system.
2. Description of the Related Art
An inverter that interconnects with an AC power system is well known. There is a case where an AC capacitor is provided on the AC side of the inverter interconnecting with the AC system, in order to control a ripple component output from the inverter.
When the system voltage drops, the amplitude of a ripple current output from the inverter will increase. Therefore, an overcurrent relay may perform a protective operation to thereby trip a circuit breaker, even if the fundamental wave component of the output current of the inverter does not reach a level that requires the protective operation. To avoid this, it is disclosed that an inverter is controlled by increasing the frequency of a carrier wave when a drop in the system voltage has been detected (see Patent Document 1). Similarly, it is disclosed that a limit value for limiting a current instruction value for the inverter is reduced when a drop in the system voltage has been detected (see Patent Document 2). Yet further, it is disclosed that a direct-current (DC) voltage applied to the inverter is increased when a drop in the system voltage has been detected (see Patent Document 3).
However, when the system voltage abruptly varies because of, for example, failure of the power system, the AC capacitor repeats charge and discharge in accordance with the variation, with the result that an over current may be output from the inverter. This over current may not be sufficiently suppressed and the circuit breaker may be tripped, if the inverter is controlled after the inverter control apparatus detects it.